1. Field of the Invention
The present invention relates to a glass powder for glass ionomer cement that is used in the medicine, and preferably in the dentistry. More particularly, the invention relates to a glass powder for glass ionomer cement having an effect for improving physical properties of glass ionomer cement, as compared with the related art glass powders for glass ionomer cement.
2. Description of the Related Art
Glass ionomer cements are used by reacting an acid made of, as the main component, an acid such as polycarboxylic acids with a glass powder for glass ionomer cement in the presence of water. The glass ionomer cements have superior characteristics such that they are extremely good in biocompatibility, translucent and superior in esthetics, have a superior adhesive force to tooth structure such as enamels and dentins, and have a caries-preventive function owing to fluoride contained in the glass powder for glass ionomer cement. For these reasons, the glass ionomer cements are a material widely used in the dentistry for filling of a caries cavity, cementing of a crown, inlay and bridge or an orthodontic band, lining of a cavity, a sealer for filling a root canal, core construction, pit and fissure sealant, and the like.
Furthermore, at present, resin-reinforced type glass ionomer cements have been developed which comprise a glass ionomer cement having a polymerizable resin component added thereto in order to prevent brittleness due to a water content during initial setting, which has hitherto been said to be a disadvantage of the glass ionomer cements, improve physical properties such as mechanical strengths and adhesion to a tooth structure, and have a superior adhesion to dental metals, resins, porcelains, etc. Moreover, glass ionomer cements in which a photopolymerization catalyst is used as a catalyst for polymerizing the polymerizable resin component and which are rapidly set by visible light are also developed, whereby more applications are expanded.
However, the glass ionomer cements are low in mechanical strengths such as bending strength and tensile strength, as compared with resin-based cements and the like. And, when a stress is applied, the glass ionomer cements have a drawback that they are readily broken due to fine voids or defects in an interior of a cement set material, cracks on a surface of the set material, and the like. This is considered to be caused by the fact that, since a matrix portion constituted by reacting a polycarboxylic acid, water, and a glass surface portion is brittle as compared with a glass portion constituted via a firm covalent bond of Sixe2x80x94O or Alxe2x80x94O and having a three-dimensional net-work structure, when a stress is concentrated into fine cracks generated in part of the set material, the cracks evade the glass portion having high strengths and are rapidly expanded in the matrix portion having low strengths, whereby the set material is broken.
For these reasons, in the dentistry, the glass ionomer cements cannot be applied for filling a cavity to which a relatively large load is applied, such as a class II cavity or a class IV cavity. Also, where the glass ionomer cements are used as a cement for bone, they have been considered to be insufficient in mechanical strengths as compared with the resin-based cements.
This invention is aimed to overcome the drawbacks of the glass ionomer cements as described above and to provide a glass powder for glass ionomer cement capable of obtaining glass ionomer cements having high mechanical strengths, particularly bending strength and tensile strength.
In order to achieve the above-described aim, the present inventors made extensive and intensive investigations on a glass powder to be used for glass ionomer cement. As a result, they paid attention to the fact that, in the related art glass powders for glass ionomer cement, after a glass raw material has been melted and cooled, the resulting product is crashed into a powdered having a mean particle size of from 0.02 xcexcm to 30 xcexcm over from several hours to several tens hours by means of a mill, whereby a shape having a major axis length and a minor axis length substantially equal to each other is obtained in a long-term processing by the mill. And, finally, it has been found that, when such the related art glass powder for glass ionomer cement is compounded with a glass powder for glass ionomer cement having a shape in which a major axis length is from 3 to 1,000 times a minor axis length, a reinforcing function in the fibrous fillers, a high effect of which is already confirmed in the field of engineering plastics, etc., is combined with a nature which the glass for glass ionomer cement having such a specific shape possesses to be reactive with an acid component, the matrix portion is reinforced, and the mechanical strengths of the glass ionomer cement set material, such as bending strength and tensile strength, can be markedly improved, leading to accomplishment of this invention.
Specifically, the present invention relates to a glass powder for glass ionomer cement comprising a glass powder for glass ionomer cement having a shape in which a major axis length is from 3 to 1,000 times a minor axis length, contained in a glass powder for glass ionomer cement. When the glass powder for glass ionomer cement according to the present invention is used, it has become possible to markedly improve the mechanical strengths, in addition to the characteristics of the related art glass powders for glass ionomer cement including the biocompatibility, the esthetics, the adhesion to a tooth structure, and the caries-preventive function.